Traditional fluid flow meters in domestic and commercial use generally include some mechanical arrangement such as a bellows, or a vane or impeller which actuates a totalising mechanism. Generally, a mechanical dial arrangement indicates the total volume of fluid that has passed through a meter. Such mechanical arrangements are not highly accurate, especially at low flow rates, such as those required to maintain a gas pilot light. Accordingly, inaccuracies in low flow rate measurement can represent a substantial loss of income to the supplier of gas, and water.
Over recent years, there have been a number of proposals that utilise electronics technology so as to provide for substantially higher accuracy of the fluid flow measurement. Such systems generally incorporate ultrasonic transducers that transmit ultrasonic signals both upstream and downstream to measure the times of flight of the signals, from which the relative speed of the fluid can be calculated. Examples of such devices for use in measuring liquid flow rates, are found in U.S. Pat. Nos. 3,898,878 and 3,329,017 which use amplitude based measurement. U.K. Patent Application No. GB 2,222,254 A identifies that amplitude measurement is generally not practical in gas flow measurement due to substantial variations In amplitude between the transducers. This results in it being difficult to detect the moment of time when a signal is received and thereby accurately determine the time of flight. GB 2,222,254 A disclosed a system which used ultrasonic signals transmitted as packets whereby a phase change is inserted into the middle of each packet, and thus represents an identifiable timing marker from which the time of flight can be calculated.
However, problems arise with the detection of a phase change when measuring fluid flow in a small duct or conduit because the ultrasonic signal is reflected by the wall(s) of the duct which causes multipath propagation. The multipath propagation effectively alters the phase relationship of energy reaching the receiving transducer, and hence, the timing of the phase change is not reliably detectable. Also, the transission of high order acoustic modes, which propagate at speeds lower than the primary wave packet, cause interference with succeeding signals. This can lead to further errors.
It is an object of the present invention to substantially overcome or ameliorate, some or all of the problems of the prior art.